Vector control schemes for series-connected six-phase two-motor drive systems

A novel concept for multidrive systems, based on utilisation of multiphase machines, has been introduced recently. It has been shown that, by connecting in series stator windings of the multiphase machines in an appropriate manner, it becomes possible to control all the machines in the group independently using vector control principles, although the whole drive system is supplied from one multiphase voltage source inverter (VSI). The concept has been investigated so far only for true n-phase machines (i.e. machines with spatial displacement between any two consecutive phases equal to 2p/n) and all the available considerations are restricted to the inverter current control in the stationary reference frame. Moreover, all the available proofs of the decoupled dynamic control within these multidrive systems are simulation based. One specific case is discussed in detail; a two-motor series-connected six-phase drive supplied from a six-phase VSI. The two-motor drive system, based on utilisation of a true six-phase machine is considered first and, in addition to inverter current control in the stationary reference frame, inverter current control in the rotating reference frame is also analysed. It is shown that this method of current control requires modifications of the decoupling voltage terms, compared to those valid for a onemotor drive, this being caused by series connection of the two machines. Next, two-motor drives based on utilisation of quasi six-phase machines (with two three-phase stator windings displaced by 301) are discussed and appropriate connection diagrams are developed. Verification of these schemes is provided by simulation. Finally, an experimental rig, which utilises a true six-phase machine connected in series with a three-phase machine and inverter current control in the stationary reference frame, is described and experimental verification of the decoupled dynamics of the two machines is provided by extensive testing.